Oil companies measure their global petroleum reserves in terms of barrels — the more, the better, as far as Wall Street is concerned.

Giant companies like Chevron Corp. and ExxonMobil carefully calculate how much oil and natural gas they can draw from the fields under their control. They constantly hunt for new reserves to replace every drop they pump from the ground. Let that reserve number slip, and risk the wrath of financiers who rate the companies on their ability to find more oil.

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